Abrasive grains, especially ceramic abrasive grains, having a defined shape and size have been known for some time.
U.S. Pat No. 5,201,916 discloses, inter alia, flat abrasive grains having, for example, a triangular, rectangular or circular shape. These abrasive grains are produced from a dispersion comprising particles convertible to α-alumina and a liquid comprising a volatile component. The dispersion is poured into a die having a flat base surface and depressions having shapes complementary to the desired shapes of the abrasive grains. Subsequently, a portion of the volatile component is removed, so as to form a precursor having the desired shape. The precursor is then removed from the die, calcined and finally sintered, so as to form the finished abrasive grain.
The abrasive grains produced by this process have two opposite base faces having essentially the same geometric shape. A longer lifetime is ascribed to the abrasive grains since small pieces are constantly breaking off from the abrasive grains during the grinding, giving rise to new cutting faces. As a result, the abrasive grains sharpen themselves. It is believed that about one to two thirds of abrasive grains having a base face in the shape of a triangle, especially of an equilateral triangle, on electrostatic scattering, are oriented such that one tip points away from the underlayer, while other abrasive grains are oriented such that the tip points toward the underlayer.
In an alternative process described in EP 615 816, elongated bar-shaped precursors are first produced by means of extrusion, and these are then divided into individual abrasive grains. The bar-shaped abrasive grains may thus have, for example, a cylindrical or prism shape.
WO 2009/085841 describes a further production process in which the precursor is dried in the die under conditions which lead to fracturing of the precursor. The fragments have, at least to some degree, surfaces and edges which are complementary to the corresponding surfaces and edges of the die and therefore have the angles defined by the die. These surfaces and edges have elevated cutting capacity. The other surfaces and edges formed by the fracturing, in contrast, are irregular.
WO 2010/077495 discloses abrasive grains having a through-opening or a non-through-opening or having a dish shape. Also described therein are production processes for such abrasive grains. Further abrasive grains having undefined openings are disclosed in WO 2010/077518. WO 2010/077491 is likewise concerned with abrasive grains having a dish shape.
WO 2010/077519 discloses abrasive grains having two opposite main faces and lateral faces that are inclined with respect to the main faces and run between them. The different lateral faces of an abrasive grain may be inclined at different angles relative to the main faces.
The document WO 2011/068724 likewise discloses abrasive grains having a base side and a tip, and also inclined lateral faces that run between them. Similar abrasive grain forms are also described in WO 2011/109188.
The document WO 2010/077509 is concerned with abrasive grains having a surface having a multitude of grooves. These grooves are produced with the aid of complementary ridges on the underside of the die.
WO 2011/068714 discloses pyramidal abrasive grains having a parallelogram-shaped, especially rhombic, base face, a kite-shaped base face and a superelliptical base face.
WO 2011/139562 discloses abrasive grains in the form of tetrahedra and modifications thereof. For example, the lateral faces may be concave or convex, the corners of the tetrahedra may be truncated, or the edges may be curved.
The abrasive grains described in WO 2012/018903 contain two or more sections in the form of plates arranged at an angle to one another.
In the process described in WO 2012/061016, an abrasive structure is first produced, containing abrasive grain precursors joined to one another via frangible connecting elements. After the sintering, the abrasive grains are separated from one another by breaking the connecting elements.
Alternatively, abrasive grains of a defined shape can also be produced by a screen printing process. This is described, for example, by WO 96/12776. This involves passing a dimensionally stable dispersion through orifices having a defined shape onto a conveyor belt and then curing said dispersion. The orifices may be present, for example, in a movable continuous belt.
A development of the screen printing process is disclosed in WO 2011/087649. In this process, the dispersion is forced through the orifices in the continuous belt by means of a pressure differential. Given suitable choice of the viscosity of the dispersion, this process can produce abrasive grains having a cross section which narrows from a first main side to a second, opposite the main side.
WO 2012/061033 describes processes for producing abrasive grains of a defined shape with the aid of laser radiation. Additionally disclosed are specific shapes of abrasive grains. For example, the abrasive grains may contain a main element and at least three bar-shaped elements that extend therefrom. More particularly, the abrasive grain may have the shape of a cross, of an uppercase letter “T”, of a star or of a Greek lowercase letter “λ”.
In general, it is assumed that abrasive grains having a defined shape have improved properties in several aspects: if the abrasive grains already have a defined shape and size on commencement of production thereof, there is no need for a subsequent sorting step by which the abrasive grains subsequently have to be divided into different size fractions. In addition, the shapes and sizes also remain unchanged between different production batches, which means that the abrasive properties have very good reproducibility. Furthermore, the abrasive grains may, for example, achieve increased total material removal, have a longer lifetime, produce an increased surface quality of the processed surface and give an abrasion outcome of better reproducibility.
Nevertheless, the abrasive grains known from the prior art have a number of disadvantages. For example, a large number of abrasive grains have to be aligned with the aid of costly and inconvenient electrostatic scattering and applied to an underlayer, in order that, for example, corners and/or edges of the abrasive grain point away from the underlayer and hence toward a surface to be processed. Without such an alignment, the material removal and/or the service life of an abrasive article would only be very low.